1. Uchenna Nwankwo, Stephan Howden and Dave Wells
Further Investigations of VDatum to NAD83 Vertical Separations Using United States Geological Service (USGS) Coastal Water Levels gage and Hydrolevel Buoy.
Uchenna Nwankwo, Stephan Howden and Dave Wells
The University of Southern Mississippi

2. Separation model infrastructure
Goal - reliable consistent (seamless) transformation among various vertical spatial referencing systems:
(a) with respect to gravity = geoid
(b) with respect to geometry = ellipsoid
(c) with respect to water level statistics = chart datum
Requirements - two sets of “separation models”:
geoid - ellipsoid separation model (classical geodesy problem)
chart datum - ellipsoid separation model (our concern here)
Tools to access separation model values – e.g. VDATUM
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3. VDATUM uncertainty in Gulf of Mexico
Published Vdatum uncertainties for MHHW, MLLW, DTL and Gt is 0.17 m while for NAVD88 is 0.08 m
“Important: Transformation Uncertainties in the ‘Louisiana / Mississippi - Eastern Louisiana to Mississippi Sound’ Regional Model, have been found to range from 20 to 50 cm in particular locations from the Mississippi River Delta north to Lake Pontchartrain. These issues most likely can be attributed to subsidence, newly established datums, and changes to the understanding of NAVD88 based on new versions of the GEOID. The VDatum Team is currently looking at resolving these uncertainties.” From
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4. Previous results at CSXRR and the Rigolets
Besides NAVD88 at the Rigolets, the other datums at both stations were within VDatum published uncertainty
All values in m
Rigolets
CSX
Datum
VDATUM
minus USM
MHHW
0.103 ± 0.171
0.089 ± 0.171
DTL
0.091 ± 0.170
0.090 ± 0.170
NAVD88
(GEOID12B)
± 0.078
0.018 ± 0.078
MLLW
0.082 ± 0.172
0.080 ± 0.172
4/20/2019
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5. Verifying USGS provided offsets at Delacroix station
NAD83 Ellipsoid
Gage datum
NAVD88
Reference Mark
GNSS antenna a e b c
 id
Parameters
Source
Delacroix (m) a
GNSS Antenna Ellipsoidal height NAD83
GrafNav8.7
/- 0.03 b
Antenna above reference mark
USM data
3.69 +/- 0.01 c
Reference mark above gage datum
USGS
4.29 +/- 0.01 d
NAVD88 seperation (GEOID 12B)
GEOID12B
/- 0.03 e
Gage datum minus NAVD88
/- 0.01 f
Gage datum ellipsoid height method A
a - b – c
/- 0.03 g
Gage datum ellipsoid height method B
d + e
/- 0.03 h
Gage datum ellipsoid height deviation
f - g
/- 0.03 d
Method B
Method A
4/20/2019
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6. Datum transfer for Delacroix station
Primary gage used Shell Beach NOAA gage
Datums are with respect to gage datum
Comparison of datum transfer results
Datum
USM 2017 (m)
NOAA (m)
NOAA 2016 (m)
DTL
1.49+/-0.06
1.45
1.43
MLLW
1.37+/-0.06
1.32
1.33
MHHW
1.61+/-0.06
1.58
1.54 Gt
0.23+/-0.06
0.26
0.21
MSL
1.44
Three months time series
JOA Surveys LLC, Online Tidal Datum Tool,
4/20/2019
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7. Datums Comparison Delacroix-VDatum
Method A
Using method A DTL and MLLW values agree to within the published uncertainty of VDatum.
Using method B DTL, MLLW and MHHW values agree to within published uncertainty of Vdatum.
From the two methods, we deduced that the VDATUM overestimates the Great Diurnal Range.
Datum
Delacroix m
NOAA Vdatum m
Delacroix – Vdatum m
DTL
/-0.06
/-0.17
-0.11+/-0.18
MLLW
/-0.06
/-0.17
-0.00+/-0.18
MHHW
/-0.06
/-0.17
-0.22+/-0.18 GT
0.23+/-0.06
0.46+/-0.17
-0.23+/-0.18
Method B
Datum
Delacroix m
NOAA Vdatum m
Delacroix – Vdatum m
DTL
/-0.06
/-0.17
-0.03+/-0.18
MLLW
/-0.06
/-0.17
-0.14+/-0.18
MHHW
/-0.06
/-0.17
-0.08+/-0.18 GT
0.23+/-0.06
0.46+/-0.17
-0.23+/-0.18
4/20/2019
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8. Datum transfer for Chandeleur East Hydrolevel buoy
Primary gage used is Dauphin Island NOAA gage
No datum transfer results from online NOAA datum transfer calculator
4/20/2019
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9. Datums Comparison Buoy-VDatum
Great Diurnal Range
 Station
Shell Beach NOAA
Pilot Station East NOAA
Pilot Town NOAA
Dauphin Island NOAA
Vdatum
SBCH
VDatum
PSE PT DI
Values
0.46
0.45
0.37
0.36
0.31
0.26
Difference
0.01
0.05
0.00
The agreement in Gt between the buoy and Pilot Station East and Dauphin Island makes our results reasonable
All the datums besides Gt do not agree to within the published Vdatum uncertainty
Underestimation of DTL by VDatum was observed to be responsible
Datum
Hydrolevel m
NOAA Vdatum m
Hydrolevel-Vdatum m
DTL
/-0.06
/-0.17
0.40+/-0.18
MLLW
/-0.06
/-0.17
0.41+/-0.18
MHHW
/-0.06
/-0.17
0.38+/-0.18 GT
0.36+/-0.06
0.39+/-0.17
-0.03+/-0.18
4/20/2019
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10. Conclusions
We suspect that the 14 cm difference in gage zero ellipsoid elevation observed at the
Delacroix station was due to subsidence
VDatum Gt compared with those from tidal datum transfers were within 0.04 m everywhere except at Delacroix where the difference was 0.23 m.
Examining Gt at the USGS gages, the buoy, and the regional NOAA tide gages, we hypothesize that the overestimation of Gt at Delacroix by Vdatum maybe due to the underlying tidal model having too much hydrodynamic connectivity to Shell Beach.
4/20/2019
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11. Conclusions
Ellipsoid to DTL agrees with Vdatum within 0.12 m, or better, everywhere but Chandeleur East HydroLevel buoy (DTL differs by ~ 0.4 m from Vdatum)
The DTL discrepancy at the buoy is preliminary. The record was only 3 weeks and was broken into 2 segments due to vandalism.
4/20/2019
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12. Future Work
Install permanent tide gage with GNSS receiver in Gulfport, MS
Deployment of buoy at the center of area of concern if funding is available.
4/20/2019
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13. Acknowledgment
Thanks to Mike Runner at the USGS for providing information about the USGS coastal gages and allowing us to perform GNSS surveys at the gages.
Funding is provided by NOAA through a grant to the USM Mapping Center of the USM Hydrographic Science Research Center.
A special thanks to Late RADM (ret.) Ken Barbor, Director of the USM Hydrographic Science Research Center
We appreciate the contributions of Marvin Story, Kevin Martin and Bryan Parker of The University of Southern Mississippi and Venice Tang of Waypoint Support, NovAtel Inc.
Nathan Wardwell of JOA surveys LLC
4/20/2019
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14. Questions?
4/20/2019
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